Microelectromechanical systems (MEMS) are miniature mechanical devices manufactured using the techniques developed by the semiconductor industry for integrated circuit fabrication. Such techniques generally involve depositing layers of material that form the device, selectively etching features in the layer to shape the device and removing certain layers (known as sacrificial layers, to release the device. Such techniques have been used, for example, to fabricate miniature electric motors as described in U.S. Pat. No. 5,043,043.
Recently, MEMS devices have been developed for optical switching. Such systems typically include an array of mechanically actuatable mirrors that deflect light from on optical fiber to another. The mirrors are configured to translate and move into the path of the light from the fiber. Mirrors that move into the light path generally use torsion flexures to translate mirror position vertically while and changing its angular from a horizontal to a vertical orientation. MEMS mirrors of this type are usually actuated by magnetic interaction, electrostatic interaction, thermal, pneumatic actuation or some combination of these. The design, fabrication, and operation of magnetically actuated micromirrors with electrostatic clamping in dual positions for fiber-optic switching applications are described, e.g., by B. Behin, K. Lau, R. Muller in “Magnetically Actuated Micromirrors for Fiber-Optic Switching,” Solid-State and Actuator Workshop, Hilton Head Island, S.C., Jun. 8–11, 1998 (p. 273–276).
When the mirror is in the horizontal position, it rests against a substrate that forms a base. Often, the mirror is subject to electromechanical forces, sometimes referred to as “stiction” that cause the mirror to stick to the substrate and prevent the mirror from moving. The same stiction forces can also prevent the mirror from being properly released from the substrate during manufacture. To overcome stiction problems, landing pads (also called dimples or bumps have been used in MEMS devices to minimize or otherwise control the contact area between the device and the underlying substrate. In the prior art, such landing pads are formed prior to deposition of a device layer either by etching pits in an underlying sacrificial layer or by depositing pads of another material prior to the deposition of the layer forming the device.
The problem of stiction with respect to an example of a MEMs mirror device 100 is shown in FIG. 1. The device 100 includes a mirror 111 formed from the device layer 112 of a substrate 110. The mirror 111 may be movably attached to the device layer by a flexure 114, actuated by an off-chip electromagnet, and individually addressed by electrostatic clamping either to a surface of the substrate 110 or to a vertical sidewall 114 of a top mounted chip 106. A first actuation force may move the mirror 111 between a rest position parallel to the substrate 110 and a position nearly parallel to the vertical sidewall 104 of the top-mounted chip 106, while the application of a second force (i.e electrostatic field) may clamp the mirror 111 in the horizontal or vertical position. The electrostatic field used to hold the mirror 111 in a position regardless of whether the first actuation force is on or off can increase the level of stiction between the mirror 111 and each landing surface.
When clamped to either the substrate 110 or the vertical side-wall surface 104, the mirror 111 may rest on a set of landing pads or dimples 122, 124, which may lie level with or protrude below or above the mirror surface, respectively. These landing pads 122, 124 may minimize the physical area of contact between the mirror 111 and the clamping surface, thus reducing stiction effects. However, since the mirror 111 and clamping surface (either the side wall 104 or the substrate 110) may be at different potentials, the landing pads 122, 124 may be made of an insulating material in order to prevent an electrical short between the mirror 111 and the clamping surface. While the insulating landing pad material does, indeed, prevent an electrical short, its inherent properties can lead to other problems. Firstly, most insulating materials have the capacity to trap electrical charge and can, in some cases, maintain that charge for long periods of time—sometimes indefinitely. As a result, the potential of the landing pads 122, 124 can drift to an arbitrary value, resulting in either parasitic clamping potential between the mirror 111 and the clamping surface, even when both are externally driven to the same voltage, or a reduced clamping force by shielding the mirror potential. Second, since the insulating landing pads 122, 124 will typically be at a potential close to the mirror potential when not in contact with the clamping surface, a rapid discharge can occur when the landing pads 122, 124 first come into the contact with the clamping surface that is a kept at a potential different than the mirror 111. This rapid discharge may be exhibited as arcing or short pulses of high current. Such surges can lead to physical damage to the landing pads 122, 124 or the clamping surface, or may produce micro-welding, where the landing pad is welded to the clamping surface—resulting in the mirror 111 being stuck.
There is a need, therefore, for a MEMS device having stiction resistant landing pads and a method of operating a MEMS device configured in a stiction reduced mode.
Modern communications systems require a level of robustness that protects the state of the optical switches from being lost in the event of a power failure. MEMS optical switches typically include an array of mechanically actuatable mirrors that deflect light from one optical fiber to another. A mirror may be retained in a specific ON or OFF state by use of an electrostatic clamping voltage. In the event of a power failure, the clamping voltage may be lost and any MEMS mirrors that were clamped in a specific state may revert to the opposing state when under the influence mechanical restoring forces. In this manner, the state of the switch may be lost in the event of a power failure.
Thus, there is a need in the art, for a method of maintaining the state of a MEMS device in the event of a power failure and an apparatus for implementing such a method.
The increasing complexity of optical switching systems has lead to development of switching fabrics that are larger than say 8×8. When scaling to such larger optical switch fabrics (e.g., 16×16, 32×32), the yield of the optical MEMS die will decrease with the increasing die size. This places a feasible upper bound on such scaling. One proposed solution to this problem is to develop a new technology with a finer pitch and, therefore, a smaller die. Unfortunately this is a lengthy development process. Another alternative solution is to use redundant mirrors on the device die. Unfortunately, this complicates the overall design of the optical switch.
It is known to tile two or more smaller dies together to form a larger device. For example, Minowa et al. uses four 4×4 arrays tiled together in a mosaic fashion to form an 8×8 array. However, for 16×16 arrays and larger, the size of the array still presents problems even if smaller devices are tiled together. For example, as the array size increases the distance between input and output fibers increases. The increased optical path between the fibers can lead to undesirable beam spreading. The beam spreading may be overcome by placing collimator lenses between the arrays. However, the alignment of the collimator lenses to the switching elements is difficult and even slight misalignment will result in optical loss that degrades switch performance. Another problem with tiling two or more dies is that the dies must be very accurately aligned with each other in order to ensure that the mirrors on one die will align with those on the other dies in the mosaic.
Prior art alignment techniques include self-alignment and active-alignment. In self-alignment, metallized bonding pads are placed on two different pieces, e.g. a MEMS device die containing rotating mirrors and a corresponding top chip. Solder is applied to the bonding pads and the two pieces are brought together such that corresponding bonding pads roughly align with each other. When solder is heated through reflow, surface tension forces between the solder and the bonding pads pull the two pieces into alignment. In active-alignment, the pieces are placed, within micron tolerances, using a pick and place tool and held in place until the solder freezes. Active-alignment allows for the use of epoxies as well as solders for attachment of the top chip to the device die. However, even using these techniques, alignment can be particularly problematic with a tiled device having four 8×8 MEMS mirror arrays totaling 256 MEMS mirrors.
Thus, there is a need in the art, for a self aligned or actively aligned optical MEMS device and a method for making it.